A orgasmic piss is similar to a peegasm it is when you finally take a piss after holding it in for an extended period of time i.e. a long trip, waiting for an end of a movie or having to finish something and just can’t get away. The feeling makes your legs weak, you feel good all over and you can’t think of anything but what is happing at that moment.

Alex "Where is Joe?"
Les "He went to the restroom he had to piss real badly."
Alex "Well I hope that teaches him not to have a Large Drink after we leave that last restroom for 40 miles."
3 Minutes later
Les “Joe feeling any better?”
Joe “Yep just had the best orgasmic piss ever.”